An ability to combine multiple light signals, each comprising different wavelengths, into a single output light signal is needed in many applications, such as quality control of produce such as fruits and vegetables, confocal microscopy, medical diagnosis, disease treatment, and waste sorting in recycling and waste management.
In an effort to satisfy this need, prior-art beam combiners based on several different technologies have been developed. These include free-space beam combiners, fiber-based beam combiners, and surface waveguide combiners based on either array-waveguide gratings or lithium niobate waveguides. Unfortunately, each of these approaches has significant drawbacks that have thus far limited their use in many applications.
Free-space beam combiners employ bulk optics elements such as mirrors, dichroic elements, filters, and lenses to bring light signals arriving on different optical paths into a single output optical path. As a result, they are bulky and expensive. In addition, the alignment of the multiple bulk elements is typically thermally sensitive and also very labor intensive. High-volume production of such systems, therefore, is difficult to do at reasonable cost and speed. Still further, the difficulty of aligning the optical components becomes increasingly difficult with each added light signal to be combined.
Optical fiber-based beam combiners employ multiple input optical fibers that are heated and then fused together to bring their cores into proper alignment with one another. The fabrication of these fusion-spliced combiners can be extremely difficult, particularly when three or more input optical fibers are desired. The difficulty arises from the way in which such combiners are made. Initially, two fibers are fused to put their cores into a close physical relationship. Once this fusion step is complete, a third fiber is added to the output portion of the first fusion splice by repeating the fusion process for the output portion and a third fiber. Unfortunately, it is extremely difficult to add the third fiber to the fusion splice without degrading the quality of overall system, since it is necessary that the complete fusion-spliced beam combiner work for all wavelengths simultaneously. In addition, for many applications, it is necessary to preserve polarization properties in the system, which is exceedingly difficult to enable with a fusion-spliced optical fiber-based beam combiner. These difficulties are exacerbated by the fact that it is typically desirable to keep the overall size of the fusion-splice region small. Further, the process becomes increasingly difficult as more and more fibers are added to the fiber-based beam combiner. As a result, high-volume production of fiber-based beam combiners is difficult. In addition, while smaller than free-space beam combiners, fiber-based combiners also tend to be too large and bulky for many desired applications.
Surface waveguide-based beam combiners have been demonstrated based on either silica-based waveguide systems comprising array waveguide gratings (AWGs) or lithium niobate waveguide-based systems. An AWG can combine many light signals that are closely spaced in wavelength and that have substantially regular wavelength spacing. They are ill suited for combining light signals having disparate wavelengths (i.e., widely spaced and irregularly spaced in wavelength), however. In addition, higher-order diffraction modes that propagate through an AWG limit the span of wavelengths for which the AWG can be used. Further, since silica waveguides require doping to define the waveguiding elements, their utility at shorter wavelengths (e.g., <600 nm) is limited. Further, silica waveguides do not tightly confine the optical mode of propagating light signals; therefore, silica-based optical-circuit elements must be large, which results in very large silica-based planar lightwave circuits.
Directional couplers to multiplex multiple light signals at telecommunications wavelengths (i.e., 1300-1600 nm) have been demonstrated in lithium niobate technology. For example, as described by R. C. Alferness in U.S. Pat. No. 4,146,297, issued Mar. 27, 1979, an asymmetric waveguide directional coupler comprising titanium-based waveguides formed in a lithium niobate substrate can be used to enable a single wavelength of light to cross over from one waveguide into the other, wherein the cross-over wavelength is determined by the relative effective refractive indices of the two waveguides. The effective refractive indices of the waveguides are controlled via an applied electric field to “tune” the cross-over wavelength to overcome variations in fabrication. Alferness further discloses a series arrangement of such directional couplers that enables the addition of one or more light signals to a bus waveguide one at a time.
Unfortunately, lithium niobate waveguides are characterized by an operable wavelength range that is relatively narrow. This is due to the reliance of dopants to form lithium niobate waveguides, which can limit their use for applications requiring shorter wavelengths. In addition, in many applications, a high extinction ratio between the TE and TM polarization modes in propagating light is desirable. It is extremely difficult to develop strong polarization dependence in lithium niobate waveguides, however. Finally, lithium niobate is notoriously expensive and difficult to fabricate relative to other waveguide technologies.